Pressure relief valves and vacuum venting valves have been used in various environments wherein some sort of chamber must be provided with a vacuum venting capability, such as during a period of operation, as well as means for relieving pressure build-up within the chamber should that be desired. One such application is in an integrated drive generator which consists of a gear differential with speed trimming hydraulic units, referred to as a constant speed drive, connected to a generator. The accessory integrated drive generator is coupled to the main aircraft engine and receives a variable speed input. The constant speed drive converts this input to a constant output speed. The mating generator utilizes the constant speed to produce a constant frequency electrical power output.
Heretofore, such integrated drive generators and similar electrical power generating systems most often have used separate valves to accomplish the "pressure relief" and "vacuum break" or venting functions. The vacuum break valve conventionally has been embodied in a vacuum break plug which operates with a sensitive or light spring design. The pressure relief valve has taken the form of various spring loaded valve assemblies which are selectively operable or actuated to release pressure within the constant speed drive and/or generator. Attempts have been made to incorporate both valves into a single assembly, but, in essence, both valves operate independently and include separately movable valve components.
Such valves are necessary for proper operation of the integrated drive generator. Specifically, during start-up of the device, vacuum is created and vacuum venting is required to equalize pressure within the device. During start-up vacuum is created by the transference of oil from the case to a pressurized external circuit and a charge pressure circuit within the integrated drive generator. The oil transfer is accomplished by the rotating pump elements. This vacuum is replaced with ambient pressure air allowed to pass over the vacuum break valve and into the case. With continued operation, air within the closed system is not allowed to exit. The oil expands due to an increase in temperature associated with operating conditions. The air within the closed system is compressed as a result of the previous events and a pressure head is created. The pressure head promotes filling the pumps within the integrated drive generator. In particular, this pressure head is used by the integrated drive generator designer to optimize the output quantities of the required high speed hydraulic pumps. The ensurance of proper pump inlet filling further serves to reduce premature life reduction due to cavitation and/or erosion of the pump elements. When maintenance or repairs are required, means must be provided to relieve the pressure head. The relief of this pressure head prevents any uncontrolled loss of fluid. Uncontrolled fluid loss, often at high temperature, from the integrated drive generator may be injurious to maintenance personnel.
The above parameters are critical when using such devices in aircraft applications. One of the major problems arises during conditions of extreme vibrations. If a valve to the integrated drive generator, such as the vacuum break valve, vibrates open, air can seep out of the device and diminish pumping capability. If the valve is constructed with separate movable components, including light spring/piston configurations, the problem of vibration can become critical. In addition, debris from a hostile environment can accumulate on the separate valve components and valve seats during the vacuum conditions of start-up and thereby cause subsequent leakage because of the extremely low seating force of the light springs.
This invention is directed to solving the above problems by providing a pressure relief valve which has a constituent vacuum vent valve component which is not subject to opening during vibrations and is not prone to accumulation of foreign matter. The pressure relief valve is biased with a spring of considerable rate to prevent unwarranted unseating during vibration. Minor debris accumulation does not inhibit the valve's ability to provide proper sealing due to the use of said spring. The valve assembly is simple in operation, cost effective and maintenance free.